The present invention relates to a method for fabricating a semiconductor device, particularly to a method for fabricating a resin-sealed semiconductor device using a leadframe provided with a dam bar made of an electrically insulating material.
An existing leadframe used for a resin-sealed semiconductor device is provided with an island to which a semiconductor device is secured, inner leads each of which is connected to a pad of the semiconductor device by a bonding wire, outer leads formed integrally with the inner leads, a frame to which the other ends of the outer leads are connected in common, and a feed hole for moving the leadframe. In this case, the inner and outer leads are integrated to form a lead and a plurality of leads are almost-radially and separately arranged on the circumference of the island. The inner leads are sealed with resin and inner ends of the inner leads are connected with the pad of the semiconductor device to form open ends. Metal of the leadframe is removed between a plurality of arranged leads, between the island and a lead, and between a suspended lead of the island and a lead. The metal is removed by means of punching by dies or chemical etching. Some leadframes have more than 100 leads. Therefore, while a semiconductor device is secured to an island and the semiconductor device and inner leads are sealed with resin, a lead may be deformed or deviated from a predetermined position. To prevent the above trouble, a dam bar is used to connect the leads so that they are not separated from each other. The dam bar is formed by leaving a die sheet integrated with a lead or the like.
A resin-sealed semiconductor device has been provided with the dam bar connecting outer leads to each other in order to keep resin when the semiconductor device is sealed with the resin and electrically insulate outer leads from each other by cutting the dam bar with a cutting die after the device is sealed with the resin. Under the above condition, because the lead width has been decreased in recent years due to decrease of the pitch between outer leads, it has been necessary to decrease the width of a cutting punch for cutting the dam bar in accordance with the decrease of the lead width. However, when the cutting-punch width is decreased, the mechanical strength of the punch is also decreased. Therefore, as the outer-lead pitch decreases, the durability and abrasion resistance of a cutting die (punch) are greatly decreased and thereby, the punching system cannot be used when the outer-lead pitch is 0.4 or 0.3 mm.
Thus, to disuse the cutting process using the punching system, a dam bar is proposed which is made by fixing an insulating resin without forming a metallic dam bar. A resin-sealed semiconductor device using this type of the insulating-resin dam bar is roughly classified into two types in view of its configuration. The first type is a type in which dam bars are kept up to an end product without being removed even after resin sealing. In the case of a leadframe of the first type, an insulating-resin dam bar is applied onto or nearby a boundary line between an inner lead and an outer lead formed on the leadframe, that is, a mold edge line serving as a mold edge after resin sealing. Thereafter, the leadframe is sealed with resin by means of resin sealing while bonding the insulating-resin dam bar. The insulating-resin dam bar uses a polyimide-based resin or a thermosetting resin. To apply a resin to a leadframe, the dispensing method is frequently used which applies resin while dropping it. After the leadframe is sealed with resin, it is passed through normal fabrication processes to form outer leads with insulating-resin dam bars left on it and formed into an end product.
The second type is a type in which an insulating-resin dam bar is removed after resin sealing by dissolving it with chemicals including solvents. In the case of a leadframe of the second type, an insulating-resin dam bar is applied onto an outer lead of the leadframe, that is, at a position slightly outside of a mold edge line by the dispensing method or the like. Then, the leadframe is sealed with resin by means of resin sealing. In the case of this type, sealing resin protrudes between the mold edge line and the insulating-resin dam bar, that is, dam resin remains between them because the insulating-resin dam bar is set outside the mold edge line so that it is easily removed later. Therefore, it is general to decrease the distance between the mold edge line and the insulating-resin dam bar in order to decrease the amount of the dam resin. After the leadframe is sealed with resin, the insulating-resin dam bar is removed by a solvent for dissolving only the insulating-resin dam bar and thereafter passed through normal fabrication process and formed into an end product. Or, the dam bar is mechanically removed.
Therefore, by using insulating-resin dam bars, the existing dam-bar cutting process is disused, problems of the durability and abrasion of a dam-bar cutting die are solved, and a resin-sealed semiconductor device is realized which can correspond to fine outer-lead pitches of 0.4 and 0.3 mm. In the case of the first type of keeping insulating-resin dam bars up to an end product, which is one of the existing resin-sealed semiconductor devices using insulating-resin dam bars, however, it is necessary to minimize the amount of insulating-resin dam bar protruded from a mold edge line. If the protruded amount of insulating-resin dam bar is too much, this causes a trouble in later lead bending because a dam bar is caught by a bent portion of a lead when forming the lead. Therefore, a problem occurs that a predetermined lead shape or dimension cannot be obtained. Moreover, in the case of this type, because dam bars are kept up to an end product, the material of an insulating-resin dam bar requires a high reliability equivalent to that of a sealing resin. Therefore, the resin cost may increase for development and practical use of the insulating-resin dam bar. In the case of the second type, that is, the type of removing dam bars after resin sealing, chemicals including solvents are used to remove insulating-resin dam bars and these solvents are deleterious substances in general. Therefore, they cause the problem of dangerousness when an operator handles them and the problem of environmental pollution. Moreover, because solvents contact a semiconductor device, the reliability of a sealing resin may be deteriorated. Furthermore, when mechanically removing a dam bar, the airtightness of a sealing resin may be deteriorated. Furthermore, in any case, a trouble such as coating irregularity or thin spot easily occurs because an insulating-resin dam bar is coated by a dropping method such as the dispensing method which is performed by dropping resin onto a lead and moreover, a problem occurs that the coating width of an insulating-resin dam bar easily becomes inaccurate or the positional accuracy of it is easily degraded.